A Thermal Protection System (“TPS”) can employ a wide variety of protection strategies. Systems may be one-time-use or re-useable. They may be monolithic (one piece) or tiled. They may employ a variety of insulation and/or cooling strategies. TPS for atmospheric entry vehicles provides one of the most challenging set of constraints. The thermal loads can be extreme, total volume and mass must be kept to a minimum, and reliability must be high. These constraints have resulted in many innovations in TPS systems over the last 50 years that have been successfully deployed in both atmospheric entry applications and other applications such as fire protection, oven and kiln insulation, and so on.
For a one-time-use TPS, one of the more effective mechanisms for rejecting heat at the surface of the TPS material is by ablation. Ablation of TPS material serves to lift the hot shock layer gas away from the surface thereby creating a cooler boundary layer. Ablation causes some of the TPS material to char and sublime through the process of pyrolysis. The gas produced by pyrolysis blocks convective heat flux. Ablation can also provide blockage against radiative heat flux by introducing carbon particulates into the boundary layer to make it optically opaque.
In atmospheric entry applications, ablation may actually provide the single most important mechanism for thermal protection. However, thermal insulation against conductive heat flow from an outer surface to an inner surface is still critical, and any effective TPS system also needs to have high thermal resistance. Preferably, it should also have low mass, which typically means that designs which minimize thickness and material density are preferred. The thickness of the material has frequently imposed a large weight penalty for a spacecraft flying a long heat pulse trajectory. For these types of missions, the prior art reusable and ablative TPS used thick refractory-fiber “tile” insulation to limit heat conduction to the structure. The TPS weight often occupied a large fraction of the total reentry vehicle weight, limiting the weight of usable scientific payload. The thickness also increased material cost. Due to a low strain-to-failure, the TPS also required a rigid support structure (if direct-bonded) or a strain isolating pad. Since tiles were both rigid and typically of significantly different thermal expansion coefficients than the underlying structure, this also means that tiles must be spaced with suitable gap fillers between them. Failure of the gap fillers can be just as serious as tile failure.
More specifically, historical ablative TPS systems (“ablators”) have been built as composite materials comprising ceramic or carbon fibers and an organic polymeric matrix. High density ablators having a density of about 1.1-1.9 g/cm3 or higher, were developed with various polymers such as epoxy, phenolic, and silicone reinforced with asbestos fibers, graphite cloth, silica cloth, etc. by known processes. Low density ablators have also been developed, for example, comprising polymers, silica or phenolic microballoons to reduce density, and ceramic fibers and/or a honeycomb structure for reinforcement. The principal method of thermal protection of these ablators was provided by ablation (i.e., pyrolysis or thermal decomposition) of the polymer.
It is important to understand the chemical processes that go on during a typical use scenario for reinforced polymer ablators. As already discussed, the polymer is a sacrificial material which is substantially pyrolized to provide cooling gases in the boundary layer. This process typically takes place in the presence of very little available oxygen, and the gas flow is outward from the ablator preventing any significant replenishment of oxygen from the surrounding atmosphere. Thus, the pyrolysis of the polymeric matrix is primarily a thermal decomposition process (which occurs because operating temperatures exceed polymer thermal stability limits), but not a burning or oxidation process. The polymer bonds are broken, releasing pyrolysis products such as oxygen, hydrogen and/or low molecular weight organic materials and leaving behind most of the carbon (at least for polymer resins comprising only carbon, hydrogen, and oxygen). The residual carbon is referred to as a “char.” Similarly, if some form of carbon filler (fibers, cloth, honeycomb, etc.) is used, that carbon survives the ablation process substantially intact. If fact, carbon turns out to be more suitable as a filler and/or matrix at higher operating temperatures than most ceramic materials, because it does not melt.
During the early years of Space Shuttle heat shield development, “passive transpiration” systems were proposed. The systems included a low density, high temperature ceramic material such as silica, carbon, potassium titanate, or graphite, impregnated with “coolants” such as polyethylene, or an epoxy, acrylic, or phenolic. Use of a passive transpiration system increases the heat rate capability of the ceramic substrate by addition of an organic coolant, which functions as a transpirant. However, the high density of the final product increased weight of the system, and because the organic coolant filled the void volume of the ceramic fiber assembly, the organic coolant acts as an additional heat conduction path, increasing the overall thermal conductivity. Conventional ablators are generally manufactured in a process wherein the polymers and other components, such as the microballoons and the reinforcing fibers, are uniformly mixed and cured. These products have a uniform density, which is also a disadvantage in minimizing weight.
More recently, low-density TPS systems have been developed, typically comprising a low-density fiber structure such as a felt made from ceramic or carbon fibers, that is “impregnated” with a polymer resin. The fiber structure was typically made with a high void fraction (more than 90%), and only sufficient resin was used to surround the fibers with resin (i.e., an amount of resin approximately equal in weight to the fiber weight). The amount of resin could also be kept low by impregnating it as a high-surface-area nanoporous material, where the nanoporous material substantially fills the voids in the fiber structure. Microscopic analysis indicated that the resin does not wet the carbon fiber, but it is nevertheless well-distributed through the fiber matrix. The finished material thus also had a high void fraction, and TPS material densities of substantially less than 1 g/cm3 could be provided which exhibited correspondingly low thermal conductivities compared to a fully filled high density material.
An important example of such a low-density ablator is the phenolic-impregnated carbon ablator (“PICA”) material developed about 15 years ago and successfully used in the Stardust mission which returned sample material from Comet Wild to Earth for analysis. PICA was also one of the leading candidate materials evaluated to make the heat shield for the next generation general-purpose launch vehicle (Orion mission). PICA was ultimately not selected, in part because the problem of finding a good gap-filling material for the space between PICA tiles was not fully solved. However PICA was chosen for the Mars Science Lab mission (MSL), due to launch in Nov. 2011 with a tiled PICA TPS.
PICA comprises a FIBERFORM™ carbon fiber backbone impregnated with a highly cross-linked phenolic polymer matrix. FIBERFORM is an anisotropic rigid material made from randomly oriented carbon fibers “glued” together with a charred resin. (Carbon fibers are themselves made by charring polymer fibers.) PICA has low density (ρ=0.23-0.27 g/cm3) and superior ablation characteristics. However, because of the rigid FIBERFORM carbon fiber substrate and highly cross linked phenolic polymer matrix used, the present formulation of PICA is a relatively weak and brittle material that has structural limitations due to its rigid nature and low strain to failure. Standard PICA must be tiled and supported by additional structural backing for applications over 1 m. PICA has poor mechanical characteristics (large variability in all mechanical properties (strength, modulus, etc.) due to its brittle behavior, and it has low “toughness” (e.g., for micrometeorite impact and general handling).
Procedures for fabrication of standard PICA material are disclosed in U.S. Pat. Nos. 5,536,562, 5,672,389 and 6,955,853, issued to Tran et al. and incorporated herein by reference.
U.S. Pat. No. 7,931,962, issued to Willcockson et al., provides a partial improvement over standard PICA by disclosing an ablator made from a flexible fabric impregnated with a flexible silicone or fluoropolymer ablator. This ablator is flexible only before use, and this enables deployable TPS designs. The use of silicone or fluoropolymer is limiting, however, in that thermal loading is limited to below 200 W/cm2. Further, the continuous structure of the impregnated polymer ablator means that the material becomes rigid and brittle after pyrolysis, which limits TPS designs to those that would be unaffected by such structural change during use. For example, silicone pyrolizes to a silica material, which is glassy and very rigid. For atmospheric entry applications, such loss of flexibility could cause catastrophic failure of a TPS system during the later stages of atmospheric entry.
What is needed is a family of new TPS materials that are flexible and conformal and have    increased toughness (less brittleness),    large strain to failure,    improved ultimate tensile strength (“UTS”),    low density, and    improved mechanical and thermal responses.
Further there is a need for materials that remain flexible when pyrolized and are capable of being applied in large sections in few pieces to minimize the need for gap fillers.